This invention relates to apparatus for the measurement of biomagnetic signals produced by the body, and, more particularly, to such apparatus in which the magnetic pickup coil may be positioned remotely from the detector.
The human body produces various kinds of energy that may be used to monitor the status and health of the body. Perhaps the best known of these types of energy is heat. Most healthy persons have a body temperature of about 98.6.degree. F. A measured body temperature that is significantly higher usually indicates the presence of an infection or other deviation from normal good health. A simple medical instrument, the clinical thermometer, has long been available to measure body temperature.
Over 100 years ago, medical researchers learned that the body also produces electrical signals. Doctors today can recognize certain patterns of electrical signals that are indicative of good health, and other patterns that indicate disease or abnormality. The best known types of electrical signals are those from the heart and from the brain, and instruments have been developed that measure such signals. The electrocardiograph measures electrical signals associated with the heart, and the electroencephalograph measures the electrical signals associated with the brain. Such instruments have now become relatively common, and most hospitals have facilities wherein the electrical signals from the bodies of patients can be measured to determine certain types of possible disease or abnormality.
More recently, medical researchers have discovered that the body produces magnetic fields of a type completely different than the other types of energy emitted from the body, but which are associated with electrical signals within the body. The research on correlating magnetic fields with various states of health, disease and abnormality is underway, but sufficient information is available to demonstrate that certain emitted magnetic fields are associated with conditions such as epilepsy. Present medical studies are investigating the nature of the normal and abnormal magnetic fields of the brain, and seeking to correlate those fields with brain functions and patient health.
For example, if it were known that a particular abnormality, such as epilepsy, were associated with an abnormal magnetic field produced at a particular location in the brain, then it might be possible to detect the abnormality at an early stage, while it was treatable, and then apply other medical knowledge to chemically treat or surgically remove that precise portion of the brain with minimal side effects on the patient. Magnetic studies of the brain therefore offer the potential for understanding and treating some of the most crippling diseases and conditions known.
The biomagnetometer is an instrument that has been developed for measuring magnetic fields produced by the body, particularly the brain. The biomagnetometer is a larger, more complex instrument than the medical instruments mentioned earlier, primarily because the magnetic fields produced by the body are very small and difficult to measure. Typically, the strength of the magnetic field produced by the brain is about 0.000000001 Gauss, at a distance of 1-2 centimeters from the head. By comparison, the strength of the earth's magnetic field is about 0.5 Gauss, or about five hundred million times larger than the strength of the magnetic field of the brain, as measured externally to the head. Most electrical equipment also produces magnetic fields, in many cases much larger than that of the earth's field. It is apparent that, unless special precautions are taken, it is not possible to make magnetic measurements of the human body because the external influences such as the earth's magnetism and nearby apparatus can completely overwhelm and mask the magnetic fields from the body.
The biomagnetometer includes a magnetic pickup coil connected to a very sensitive detector for magnetic signals. The currently most widely used detector is a Superconducting QUantum Interference Device or SQUID, which, in combination with a superconducting pickup coil, is sufficiently sensitive to detect magnetic signals produced by the brain. (See, for example, U.S. Pat. Nos. 4,386,361 and 4,403,189, whose disclosures are incorporated by reference, for descriptions of two types of SQUIDs.) The detector, pickup coil, and their associated equipment require special operating conditions such as a cryogenic dewar, and cannot be placed into the body or attached directly to the surface of the body. The dewar is maintained at liquid helium temperature (about 4.2K), to maintain the SQUID detector, the pickup coil, and the electrical connection between them in the superconducting state because of the small electrical currents involved, and to reduce the electrical noise that might otherwise influence the SQUID detector.
The present biomagnetometer therefore includes a dewar structure in which the pickup coil, the SQUID detector, and the electrical interconnect are immersed. The dewar normally is constructed with a tail (see U.S. Pat. No. 4,773,952, whose disclosure is incorporated by reference, for a description of the construction of the dewar), which permits placement of the pickup coil in proximity with the head of the patient, typically about 2 centimeters away. Special electronics is provided to filter out external effects such as the earth's magnetic field and the magnetic fields of nearby electrical instruments. (For a description of such a device, see U.S. Pat. Nos. 3,980,076 and 4,079,730, whose disclosures are herein incorporated by reference.) The patient and detector can also be placed into a magnetically quiet enclosure that shields the patient and the detector from the external magnetic fields. (For a description of such an enclosure, see U.S. Pat. No. 3,557,777, whose disclosure is herein incorporated by reference.) With these special precautions, medical researchers and doctors can now make accurate, reliable measurements of the magnetic fields produced by the brain, and are studying the relationship of these fields with diseases and abnormalities.
The existing approach of enclosing the pickup coil and the SQUID detector in a liquid-helium dewar is acceptable in many circumstances. Nevertheless, it would be desirable to have another approach wherein the bulk could be reduced, to permit more flexibility in the use of the biomagnetometer. It would also be desirable to permit changes in the pickup coil configuration without removing the coil from the dewar, which necessitates a restabilization and recalibration of the system when the new coil is immersed back into the dewar. The present invention fulfills these needs, and further provides related advantages.